C  H 
431 
A7 
Biology 


UC-NRLF 


C   2   b72    373 


LIBRA 

UNIVER 
OF 

CALIFO1 


HORNS  IN  SHEEP  AS  A  TYPICAL  SEX-LIMI- 
TED CHARACTER 


T.  R.  ARKELL  AND  C.  B.  DAVENPORT 


[Reprinted  from  SCIENCE,  N.  S.,  Vol.  XXXV.,  No.  897,  Pages  375-377,  March  8, 


[Reprinted  from  SCIENCE,  N.  S.,  Vol.  XXXV.,  No.  897,  Pages  375-377,  March  S,  1912] 


HORNS  IN  SHEEP  AS  A  TYPICAL  SEX-LIMITED  CHARACTER1 


SEVERAL  years  ago  Wood  (1905)  published 
a  note  in  -which,  he  showed  that,  in  a  cross  be- 
tween a  Dorset  Horn  and  a  Suffolk  (belong- 
ing to  a  hornless  breed  of  sheep),  the  male 
offspring  all  developed  horns  but  the  female 
offspring  remained  hornless.  He  showed 
further  that  in  the  F2  generation  hornless 
males  arise,  and  these  do  not  carry  the  de- 
terminer for  horns,  and  horned  females,  but 
only  when  they  have  the  determiner  duplex. 
Bateson  (1909,  p.  173)  has  discussed  these 
facts  and  drawn  the  conclusion :  "  Sex  itself 
acts  as  a  specific  interference,  stopping  or  in- 
hibiting the  effects  of  a  dominant  factor,  and 
it  is  not  a  little  remarkable  that  the  inhibi- 
tion occurs  always,  so  far  as  we  know,  in  the 
female,  never  in  the  male."  He  admits,  how- 
ever, the  difficulty  in  distinguishing  between 
this  probability  and  the  other  possibility;  viz., 
that  the  male  provides  a  stimulating  factor. 
Castle  (1911,  p.  102)  concludes  that  the  rea- 
son horns  are  more  strongly  developed  in 
males  than  females  is  "the  presence  of  the 
male  sex-gland  in  the  body,  or  rather  prob- 

1  Joint  contribution  from  the  New  Hampshire 
Agricultural  Experiment  Station  and  the  Station 
for  Experimental  Evolution,  Carnegie  Institution 
of  Washington. 


ably  some  substance  given  off  into  the  blood 
from  the  sex  gland,  favoring  growth  of  the 
horns  " ;  and  he  adds  that  if  the  male  Merino 
sheep  (in  which,  usually,  the  male,  and  the 
male  only  is  horned)  is  castrated  early  in  life 
no  horns  are  formed.  He  gives  no  reference 
for  the  last  statement;  and  in  view  of  thep- 
variability  of  the  horned  condition  in  the 
males  of  the  "  Merinos  "  the  conditions  of  the 
experiments  would  have  to  be  carefully  con- 
sidered before  such  a  result  could  be  accepted 
as  settling  the  question  of  the  dependence  of 
horns  in  heterozygous  males  upon  a  secretion 
from  the  testis. 

The  hypothesis  that  we  have  adopted  and 
which  works  with  entire  satisfaction  assumes, 
first,  that,  as  in  man  so  in  sheep,  the  male  is 
heterozygous  (simplex)  in  sex.  One  sex- 
chromosome  is  then  to  be  expected  in  the  male, 
and  substantially  this  condition  has  been 
found  to  hold  for  man  by  Guyer  (1910).  The 
female  will  then  be  duplex  in  respect  to  sex. 
One  further  assumption  is  necessary;  there  is 
an  inhibitor  to  horn  formation,  and  this  is 
located  on  the  sex  chromosome;  consequently 
it  is  simplex  in  the  male  and  duplex  in  the 
female.  Thus  it  belongs  to  the  well-known 
class  of  sex-limited  characters.  The  inhibi- 


272702 


Determiners  in  Germ  Plasm  of 

No.  of  Horned  and  Hornless  Offspring 

Male 

Female 

Males 

Females. 

Horned 

Hornless 

Horned 

Hornless 

Soma       Xxhhli  (hornless)     

XXhhll  (hornless) 
Xhl 

XXhhll  (hornless) 

0 

4 

0 

12 

Gametes  Xhl.  

xhi 
Zyjrotes   Xxhhli  (hornless)  

fioma       Xxhhli  (hornless)  

XXH  h  II  (hornless,  simplex) 
XHI 
Xhl 
XXHhII(  hornless) 
XXhhll  (hornless) 

(2) 
2 

(2) 
2 

(0) 
0 

(2) 
2 

Gametes  Xhl  .        

Xxhhli  (hornless)  

Soma       Xxhhli  (hornless)  

XXHHII  (  horned) 
XHI 

XXHhll  (hornless) 

15 

0 

0 

24 

Gametes  Xhl.  

xhi 

Soma       XxHhli  (horned,  simplex)  

XXhhll  (  hornless) 
Xhl 

XXHhll  (hornless) 
XXhhll  (  hornless) 

(.5) 
0 

(.5) 

(0) 
0 

(1> 

Xhl 
xHi 
xhi  . 
Zygotes  XxHhli  (horned)  

Xxhhli  (  hornless  )  

Soma       XxHhli  (horned,  simplex)  
Gametes  XHI  

XXHhll  (hornless,  simplex) 
XHI 
Xhl 

XXHHII  (horned) 
XXHhll) 
XXHhll}-  (hornless) 
XXhhll  J 

•  (7.5) 
6 

(2.5) 

4 

(2) 

(6) 
7 

Xhl  

xHi 
xhi 
Zygotes  XxHHIi}  

XxHhli  }•  (horned)  

XxHhli  j  

Xxhhli  (hornless)  

Soma       XxHhli  (horned,  simplex)  
Gametes  XHI  

XXHHII  (horned) 
XHI 

XXHHII  (horned) 
XXHhll  (hornless) 

(10) 
10 

(0) 
0 

(4) 
4 

(4) 
4 

Xhl 
xHi 
xhi 
Zygotes  XxHHIi  (horned)  

Xx  Hhli  (  horned  )     

Soma       XxHHIi  (horned)   ...,  

XXhhll  (hornless) 
Xhl 

XXHhll  (hornless) 

5 

0 

0 

8 

Gametes  XHI  

HJH 

Soma       XxHHIi  (horned)  

XXHHII  (horned) 
XHI 

XXHHII  (horned) 

6 

0 

14 

0 

xHi 
Zygotes   XxHHIi  (horned)  

tor,  then  (designated  in  the  table  by  the  letter 
I,  its  absence  by  i),  will  always  be  double  in 
the  female  and  single  in  the  male  and,  in  the 
gametes,  will  always  be  associated  with  the  sex- 
chromosome,  which  is  designated  through- 
out by  the  symbol  X;  its  absence  by  x.  In 
the  zygote  the  single  inhibitor  is  incapable  of 
preventing  the  development  of  the  determiner 
for  the  horn  (JET)  even  when  the  latter  is  only 


simplex.  But  the  double  inhibitor  is  capable 
of  preventing  the  single  horn  (HJi)  deter- 
miner, but  not  the  double  determiner  (HH~). 
The  table  gives  a  summary  of  matings 
used,  their  hypothetical  somatic  and  gametic 
composition,  and  the  proportion  of  each  sort 
of  zygote  that  will  be  formed  in  each  sex. 
The  actual  frequency  of  offspring  derived 
from  each  mating  is  given  on  the  left  of  the 


SCIENCE 


table;  the  expected  proportions  in  the  more 
complex  cases  being  given  above  the  actual 
findings  in  parenthesis.  The  matings  were 
made  and  the  offspring  examined  in  major 
part  at  the  New  Hampshire  Agricultural  Ex- 
periment Station  and  in  minor  part  at  the 
Station  for  Experimental  Evolution.  The 
latter  station  was  able  to  contribute  especially 
to  the  results  of  later  generations.  For 
horned  females,  Dorsets  were  used ;  for  horned 
males  Rambouillets,  Dorsets  and  the  Scottish 
4-horned  race.  As  hornless  races  the  Downs 
were  chiefly  employed.  It  is  not  our  purpose 
now  to  give  complete  details,  as  the  experi- 
ments are  being  continued  and  full  data  will 
be  deferred  until  the  publication  of  our  final 
report. 

The  results  of  the  table  accord  very  closely 
with  expectation,  so  that  we  are  justified  in 
concluding  that  an  explanation  of  the  results 
like  that  we  offer  is  the  correct  one.  By  our 


formula,  then,  the  case  of  inheritance  of 
horns  in  sheep  is  brought  quite  into  line  with 
that  of  other  sex-limited  characters,  its  pecul- 
iarities being  due  to  an  inhibitor  of  horn  de- 
velopment that  is  carried  in  the  sex-chromo- 
some. 

LITERATURE  CITED 

Bateson,  W.,  1909,  "Mendel's  Principles  of  He- 
redity," Cambridge,  Eng.,  University  Press. 

Castle,  W.  E.,  1911,  "Heredity  in  Belation  to  Evo- 
lution and  Animal  Breeding,"  New  York,  Ap- 
pleton. 

Guyer,  M.  F.,  1910,  "Accessory  Chromosomes  in 
Man,"  Biol.  Bull,  XIX.,  219-234,  PI.  I. 

Wood,  T.  B.,  1905,  "Note  on  the  Inheritance  of 
Horns  and  Face  Color  in  Sheep,"  Jour.  Agrio. 
Sri.,  I.,  364,  365,  PI.  IV. 

T.  E.  ARKELL, 
C.  B.  DAVENPORT 
DURHAM,  N.  H., 
COLD  SPRING  HARBOR,  N.  Y., 
January  29,  1912 


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